Topology discovery is a critical issue in order to have a well managed Ethernet network. There are some approaches in the literature that aim to solve this task, nonetheless, each of them has significant disadvantages.
Link Layer Discovery Protocol (LLDP), described in IEEE 802.1AB—Station and Media Access Control Connectivity Discovery, which is the emerging IEEE standard 802.1AB, promises to simplify troubleshooting of enterprise networks and enhance the ability of network management tools to discover and maintain network topologies. The protocol LLDP is a neighbor discovery protocol. It defines a standard method for Ethernet network switches to advertise information about themselves to neighbor nodes on the network and store the information that they discover. They store the LLDP information in an IEEE-defined Simple Network Management Protocol (SNMP) Management Information Base (MIB), thus it is available to network management systems. Nevertheless, LLDP proposes a standard topology discovery approach, LLDP may/will not be implemented in all Ethernet switches, especially not in low-cost products.
A serious drawback of the protocol LLDP is that it is not available in low-cost Ethernet switches. Therefore an operator having some switches that do not support LLDP cannot have topology discovery based on this protocol. This is true especially when each switch is a low-cost node without LLDP in order to have a low-cost network. Authors of R. Black, A. Donnelly and C. Fourne, “Ethernet Topology Discovery without Network Assistance,” 12th IEEE International Conference on Network Protocols (ICNP'04), 2004, pp. 328-339, propose an Ethernet (Layer 2) topology discovery scheme without assistance from the network elements but relying on the hosts connected to the network. Most hosts in the network run a daemon that injects suitable probe packets and observe where they are delivered. Therefore, network hosts require new functionality, which does not fit into a low-cost approach.
The most significant drawback of this is that each host has to run a daemon in order to achieve topology discovery, which is hard to accomplish in an Ethernet network, especially in a low-cost one. Further problem of probe packet based approaches is that they can only discover the forwarding topology but not the physical topology. That is physical links that are actually not used for packet forwarding, e.g. blocked by STP, cannot be discovered in a probe packet manner.
Another approach is proposed in M-H. Son, B-S. Joo, B-C Kim and J-Y Lee, “Physical Topology Discovery for Metro Ethernet Networks,” ETRI Journal, vol. 27, no. 4, August 2005, pp. 355-366. This approach is based on monitoring of Spanning Tree Protocol (STP) messages, i.e. Bridge Protocol Data Units (BPDU) and retrieving STP MIBs from the switches.
The above approach proposed by M-H. Son et al cannot be applied in a network where STP is disabled for some reason, e.g. TRILL or it is assured by VLAN design that the topology is loop free. Furthermore, the assumptions used therein are not valid for all switches, i.e. it is vendor dependent.
In B. Lowekamp, D. R. O'Hallaron and T. R. Gross, “Topology Discovery for Large Ethernet Networks,” ACM SIGCOMM 2001, San Diego, Calif., USA, August 2001, pp. 237-248, and in Y. Bejerano, Y. Breitbart, M. Garofalakis and R. Rastogi, “Physical Topology Discovery for Large Multi-Subnet Networks, “IEEE INFOCOM 2003, San Francisco, USA, April 2003, pp. 342-352, Address Forwarding Table (AFT) information is collected via SNMP MIBs to discover the topology, but only spanning tree paths can be retrieved.
A common feature of those works described in the two above publications is that they only use AFT entries. So, they can only find spanning tree paths and exclude multiple redundant paths. Even though their techniques are said to discover the physical topology of Ethernet networks, they have the limitations of discovering only Layer-2 spanning tree paths.
To summarize: existing solutions, that do not use LLDP and are claimed to discover the physical topology, only discover the forwarding topology determined by STP or in some cases able to find links that are blocked by STP but they do not discover all physical links in a heterogeneous Ethernet network.